Well, the size does not matter that much as long as it does not get significantly bigger than it is now. Two things that do matter in my opinion are:

- Display output: Our monitors are still mostly VGA, so this needs to work reasonably with a cheap adapter

- Power: Ideally it would be Apple TV-like, i.e. one voltage, significantly lower than 12V, but anything civilized will do (standard voltage such as 12V, 18.5V etc with no digital communication between the computer and the power brick)